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Full text of "The manufacture and properties of iron and steel"

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THE BASIC OPEN-HEARTH PROCESS.                         191
the basic furnace is the melting and decarburization of iron as in acid practice, with the additional duty of removing a reasonable quantity of phosphorus and some sulphur. Under the influence of the flame and ore, the phosphorus is converted into phosphoric acid (P205) which can unite with iron oxide, but the conjunction will be only temporary, for the carbon of the bath reduces the iron, and then the phosphorus in its turn is robbed of its oxygen and returned to the bath. But if lime is added, the acid can form phosphate of calcium, and since the oxide of this element cannot be reduced by the carbonic oxide, the phosphorus is never left without a partner, but forms part of a stable cinder. This oxide of calcium is sometimes added in the form of limestone, the carbonic acid being expelled in the furnace. This entails a considerable absorption of heat, and the melting must be delayed accordingly; but it has a compensating advantage in that the gas, in bubbling through the metal, keeps up a motion which facilitates chemical action, and also that the carbonic acid gives up part of its oxygen to the silicon, phosphorus, carbon and iron.
This oxidizing action allows the use of a greater proportion of pig-iron, and aids in the removal of phosphorus, so that there seems to be good ground for using the stone in its natural state. I believe, however, that it is more economical to put it through a preliminary roasting and reduce by nearly 50 per cent, the amount of basic addition, for the rate of melting is thereby hastened, while the oxidizing effect can be obtained by the use of ore. Ore costs more than stone, but its full value is returned in metallic iron, and, moreover, it is possible to use a greater proportion of pig-iron on account of the reduced quantity of gas evolved, for the oxidation done during melting, either by stone or ore, is limited by the frothing of the stock, and this is determined by the amount of gas evolved in the reactions. Therefore, if ore produces less gas than stone in oxidizing a given quantity of carbon, then more pig can. be used with ore than with stone. The reactions are as follows:
Limestone, CaCOĢ+O=2 CO+CaO. Ore, FeaOs+3 C=3 CO+2 Fe.
Thus, two volumes of gas are formed for each atom of carbon when stone is used, while only one volume is produced with ore. The available oxygen in the ore is nearly twice as much as in